Set maximum field for name. The max field contains the number of days that the password is valid for name. The aging for name will be turned off immediately if max is set to -1. If it is set to 0, then the user is forced to change the password at the next login session and aging is turned off.

another option:

-f

Force the user to change password at the next login by expiring the password for name.

but when I log into xp it does not ask me to change the password. Surely passwd changes te linux user and not the samba one? Also passwd -f just asks for a new password at the command prompt on the server.